1. Field of the Invention
The invention relates to a combination radio-dead reckoning system for the accurate guidance of moored vessels, for example, for undersea pipe or cable laying and more particularly relates to such a system for supplying position data when radio transmissions are absent. The system solves the problem by the manipulation of a number of parameters derived from sensors normally aboard a conventional pipe laying vessel and sensors cooperating with active winches applying tension to its several anchor lines. The vessel's pitch, roll, and heading angles are supplied from a marine gyrocompass and a vertical gyroscope. Anchor line elevation and azimuth angles with respect to deck coordinates are yielded by conventional angle pick offs associated with fairlead devices, as are the line axial tension and the length of anchor line paid out. Initial conditions, including the initial position of the vessel, are easily obtained in the usual manner. From certain of these parameters, the invention derives intermediate anchor line parameters for use in computing fairlead motion.
The intermediate data is used to compute compensated fairlead motion data by first computing apparent fairlead motion from anchor line tension changes. This version of fairlead motion contains undesired terms due to apparent fairlead motion caused by anchor line length changes and also due to actual fairlead motion caused by vessel attitude changes. Therefore, second and third computations are made of the contributions of the latter effects. This is done so that apparent fairlead motion due to anchor line length change and actual fairlead motion due to vessel attitude change may be subtracted from the measure of apparent fairlead motion as calculated from anchor line tension change, yielding the desired compensated fairlead motion value free of significant disturbing components. An estimated value of vessel position change is then obtained by least squares estimation involving these fairlead motion terms. Correction of vessel position then follows by cooperative operation of the winch and thruster assemblies.
2. Description of the Prior Art
Before an undersea pipe-lay campaign is begun according to past practice, a survey is made of the area involved. Based on bottom profiles made with depth sounders, a suitable route is determined for the pipe and is mapped. As the pipe is being laid, actual pipe position is drawn on the map by operators on board the pipe-lay vessel. The pipe position is determined by combining the known value of vessel position and the pipe touch down location relative to the vessel. This later can be determined by several methods, one of which is by the use of an underwater vehicle with television cameras.
After several sections of pipes are laid, vessel position is determined using a suitable radio positioning device. The actual and desired pipe routes are examined, and a determination is made as to the direction in which the vessel is to be moved. The winch operators are then asked to correct the position error by moving the vessel in the proper direction over the next several warps. The operators then cause the vessel to be warped or moved in the desired direction one section-length so as to line up the pipe joints in the weld stations, and then welding of another section of pipe is started. This continues until the pipe is once again being placed on the proper route.
The position determination as described above is done after every pipe section is laid, or less frequently depending on the depth and the condition of the sea-bed. The operators move the vessel using the winches or thrusters. They may use winches only, thrusters only, or winches and thrusters together.
Control of the winches may be performed by several operators, for example, three:
1. the forward winch operator, handling the six forward winches using one hand-controller for each winch, PA1 2. the aft winch operator, for the six aft winches, there again being one-hand controller for each winch, and PA1 3. the winch director, who has a console which can control all winches, again using one hand-controller for each winch.
Control of the winches has been performed by the forward and aft operators cooperating as a pair, or by the winch director alone.
The control of six winches is difficult and requires experience. In general, an operator (the forward winch operator, for example), is commanding only four winches, because normally not all six anchors are set. Two of those in use are generally placed more or less along the pipe-route; i.e., forward and the other two (or one) to the side. Thus, to move forward along the pipe route, the operator generally needs only to command two of the winches. The aft winch operator similarly controls his winches. Sideways movements and movements necessitating heading changes are more complex. The thruster operator, who is distinct from the three winch operators, controls all four thrusters. He can command the thrust (forward or reverse) and azimuth angle (360.degree.) of the thrusters as pairs only. That is, the forward two thrusters always thrust together in the same direction, and the aft thrusters likewise.
There are many problems associated with manual control. Operation is satisfactory in relatively shallow water, since the cables are then tight, and the winches respond relatively rapidly to operator commands. However, as depth increases, winch response becomes increasingly sluggish due to the long length of cable involved and the attendant decrease in the effective spring constant of the cable, making manual control difficult. With increasing depth or adverse sea-bed conditions, a position determination is required more and more often so as to chart the pipe route accurately. Thus, a reliable method of position determination becomes a requirement.
Accurate knowledge of the pipe location depends upon accurate knowledge of vessel position. Two known types of radio positioning systems for locating the vessel itself with sufficient accuracy are the line-of-sight radio ranging system, which obtains position by measuring range to two shore stations whose positions are accurately known by survey, and a lower frequency non-line-of-sight system that generates two hyperbolic lines of position by measuring phase differences between the signals received from pairs of shore stations. This latter method is very similar to Loran except that the frequency is higher and accuracy is better.
The line-of-sight system is the more accurate of the two systems and is capable of operation 24 hours a day. However, range capability is rather small, and operation outside of the range of the shore stations cannot be achieved. In addition, rain adversely affects operation.
The hyperbolic system effectively eliminates the range problem, although its accuracy is somewhat less than the line-of-sight system. However, it suffers from erratic operation, or no operation at all in hours of twilight and dark due to changes in the ionosphere which affect the transmission of the radio signals required. In addition, movement of large cranes on-board the vessel adversely affects the medium frequency hyperbolic system. This erratic operation presents serious problems during any campaign which requires a substantially continuous knowledge of vessel position. If automatic operation is used, as is expected to be necessary with increasing depth and adverse sea-bed conditions, continuous positional data is a requirement. Thus, any cessation of radio navigation data is a serious matter.
The fact that a radio navigation system that can supply accurate positional fix data is used on-board the vessel, and that this type of navigation system normally stops operating periodically, suggests the additional use of a dead-reckoning system to fill in the gaps between periods of radio position data, either of the velocity dead-reckoning or the inertial dead-reckoning types. In velocity dead-reckoning, the last good radio position data is extrapolated using vessel velocity as measured by a speed log affixed to the vessel. Velocity is integrated, thus giving position change from the last radio position, and hence a value of vessel position. Unfortunately, any error in velocity will also be integrated, causing a build-up of position error. These velocity errors can be due to speed log sensor errors or changes in water currents. The latter causes errors since the speed logs measure vessel velocity with respect to the water, and water current must be removed to obtain vessel ground velocity for integration. Even under the best of circumstances, a dead-reckoning scheme of this sort will cause error build-up of sufficient magnitude to affect pipe-lay operation adversely in a period of minutes, while the radio data outage is expected often to last overnight.
The error build-up is a funtion of time since this dead-reckoning process is time integration. The pipe-lay vessel, however, moves very slowly; i.e., one length of pipe (about 12 to 24 meters) in anywhere from ten minutes to an hour depending on the diameter of pipe being laid and, hence, the amount of welding that must be done. Thus, dead-reckoned position error build-up will reach sufficient magnitude to affect operation before very many sections of pipe can be laid. During critical conditions, this can even require operation to cease.
The inertial system is an improvement over the dead-reckoning scheme; but, it also is a dead-reckoning system wherein vessel velocity is obtained by integration of accelerometer data and, hence, position by a further integration. Disadvantages of this system are first its very large penalty in terms of initial cost, and then in terms of maintenance requirements and their cost. In addition, this dead-reckoning process is also a time integration as is the first scheme and, even though better than the former, simply slows down the error build-up. A high quality marine inertial navigator has an error build-up of about 70 meters per hour, which is unacceptable for pipe laying operations.